Question

I’m trying to work on a regex js username checker for registration, however I’m having a few issues

This is the expression I wrote

^[a-zA-Z]{1}([a-zA-Z0-9]*[\._ ]?){2,10}[A-za-z\d]$

The problem is that it doesn’t limit the period, underscore, and spaces, and also the username should be between 4-12 characters.

I’m trying to allow:
First character has to start with A-Z a-z
Last character has to be alphanumeric
Everything inbetween must be 2-10 alphanumeric characters and allowing only 1 space, period and underscore.

For example:

• A___N is not valid (3 undescores)
• A_NON is valid
• “A.Non ” is not valid, space at the end.
• A.non is valid
• A N.oN is valid

Any help would be appreciated

Answer 1

The best approach is to use a lookahead assertion to combine two unrelated requirements. Essentially, you need ensure that this regex does match:

^[a-zA-Z][a-zA-Z0-9._ ]{2,10}[a-zA-Z0-9]$

and that this one does not match:

([._ ]).*\1

You can combine them by using a negative lookahead assertion:

^(?!.*([._ ]).*\1)[a-zA-Z][a-zA-Z0-9._ ]{2,10}[a-zA-Z0-9]$